The present invention relates to a conductive silver paste and a conductive film such as a conductor wiring having a fine plane shape formed using the same.
A conductive silver paste including silver powders is printed on a surface of a base material using various types of printing methods, or is applied thereon using various types of coating methods, dried, and is further heat-treated as required, thereby forming a conductive film such as a conductor wiring.
Generally used as the conductive silver paste is one containing thermosetting resin, its curing agent, silver powders, and a solvent. As the silver powders, ones having various shapes are used. Particularly in order to reduce contact resistance between the adjacent silver powders in a conductive film to improve the conductive properties of the conductive film, flake-shaped silver powders are suitably used.
However, any of the conventional silver powders are large-diameter ones having an average particle diameter of not less than 1 μm, and a filling ratio represented by a volume ratio of the silver powders in the unit volume of the conductive silver paste is low. Even if the contact resistance is reduced by changing the shape of the silver powders into a flake shape, therefore, the effect of improving the conductive properties of the conductive film has a limitation, and a further improvement in the conductive properties is required.
As various types of electronic equipment are miniaturized, it is also required that the conductor wiring composed of the conductive film is made fine. For example, in a conductor wiring having a line shape, it is required that the line width of the conductor wiring and the space width between adjacent conductor wirings are respectively not more than 100 μm. In the future, it is predicted that the conductor wiring is required to be made finer. However, the conventional large-diameter silver powders have not been able to sufficiently cope with the formation of the conductor wiring having such a fine plane shape.
When the above-mentioned conductor wiring is formed by a screen printing method using the conductive silver paste, for example, a screen having a screen opening sufficiently smaller than the line width of the conductor wiring and the space width between the conductor wirings must be used in order to satisfactorily reproduce the fine plane shape.
However, such a screen having a small opening is liable to be clogged with large-diameter silver powders having an average particle diameter of not less than 1 μm. When the screen is clogged, there occurs such inferior printing that the printed conductor wiring is scratchy and a line of the conductor wiring is broken halfway. Particularly in an edge portion of the conductor wiring, the granularity of sufficiently larger silver powders than the fine plane shape is noticeable, so that there also occurs such inferior printing that it is recognized that the edge portion of the conductor wiring is blurred.
Therefore, a silver compound paste containing minute silver oxide powders having an average particle diameter of not more than 500 nm and a tertiary fatty acid silver salt is proposed in place of the large-diameter silver powders (Japanese Patent Publication No. JP, 2003-203522, A).
When printing is done using the silver compound paste, and heat-treatment is then performed at temperatures of 150 to 250° C., silver oxide is reduced to silver, and powders of the silver obtained by the reduction is welded by deposited silver formed by the decomposition of the tertiary fatty acid silver salt, so that a continuous silver conductive film is formed.
When the tertiary fatty acid silver salt, together with the silver oxide, is kneaded to prepare the silver compound paste, it also functions as a lubricant to more finely grind the silver oxide and stably disperse powders of the ground silver oxide in the paste.
Even if the paste does not contain resin such as thermosetting resin, therefore, the silver oxide powders and fatty acid silver salt can be turned into paste. Combined with the fact that the silver oxide powders are minute, as described above, so that the filling ratio therewith can be increased, therefore, the conductive properties of the conductor wiring or the like can be more significantly improved than ever before. Further, the particle diameter of the silver oxide is small, and the silver oxide powders are further made minute in the paste by being kneaded with the tertiary fatty acid silver salt, which can sufficiently cope with the formation of a conductor wiring having a fine plane shape, as previously described.
In order to apply the paste to various types of printing methods and coating methods to form the conductive film such as the conductor wiring, however, it is preferable that resin and particularly, thermosetting resin is contained in the paste. The reason for this is that the resin has functions such as a function of preparing the viscosity of the paste in a range suitable for various types of printing methods and coating methods and a function serving as a binder of binding silver powders in the conductive film after formation to form on a surface of a base material a conductive film, which firmly adheres to the base material, superior in durability.
Therefore, it is examined that the silver oxide powders are turned into paste by being blended with the thermosetting resin, the curing agent, and the solvent. However, the silver oxide releases active oxygen to transform the thermosetting resin, thereby causing various types of abnormalities in the paste.
That is, the thermosetting resin is easily transformed and gelated by the active oxygen released from the silver oxide. If the thermosetting resin is gelated, therefore, the paste loses its uniform fluidity so that the screen is easily clogged. When the conductive film is a conductor wiring, therefore, there occurs such inferior printing that the printed conductor wiring is scratchy and a line of the conductor wiring is broken halfway.
The active oxygen prevents the curing reaction of the thermosetting resin and particularly, epoxy resin. Therefore, the function serving as a binder, described above, cannot be obtained. On the surface of the base material, the conductive film, which firmly adheres to the base material, superior in durability cannot be formed.
Japanese Patent Publication No. JP, 2002-29983, A discloses that a paste containing silver powders having an average particle diameter of 1 to 100 nm whose surfaces are processed by dispersants such as amine, alcohol, or thiol dispersants and thermosetting resin is prepared, the paste is printed or applied over a surface of a base material, followed by heating, to subject the thermosetting resin to curing reaction as well as sinter a lot of silver powders to form a conductive film. If such minute silver powders are used, it is possible to form a conductor wiring or the like having a fine plane shape which can also sufficiently cope with future requirements.
The surfaces of the silver powders are processed using the dispersant for the following reasons. That is, the minute silver powders as described above, are high in surface activity. Therefore, the silver powders are agglomerated in the paste before printing, are melted and integrated at approximately room temperature to easily produce agglomerated particles when the surfaces are not processed using the dispersant. The agglomerated particles produced by melting and integrating a large number of silver powders have a large particle diameter and have an indeterminate form. Particularly when the conductive film is a conductor wiring, therefore, the screen is clogged and correspondingly, is easily scratchy and broken. Further, an edge of the conductor wiring is also easily blurred. On the other hand, the silver powders processed using the dispersant are not agglomerated and are uniformly dispersed in the paste, so that such inferior printing may not occur.
If the minute silver powders are used, as described above, printing properties are improved, and a filling ratio is also improved. However, contact resistance between the silver powders is increased, and the effect of improving the conductive properties of the conductive film is not obtained.